For glass covers to be used with mobile phones or tablet terminals, a thin glass sheet before or after being chemically tempered is employed. These glass sheets are cut from a large glass sheet in a shape slightly larger than the final shape, and then the outer periphery is cut, and holes are bored therein, for example, at loudspeaker and button positions.
Patent Literature 1 discloses a reduced-diameter grinding tool which is provided with a cylindrical reduced-diameter processing section having the surface to which diamond particles are adhered, and a shaft secured to a chuck (for example, see FIG. 11). Patent Literature 2 discloses a chamfering device for chamfering by pushing a chamfering drill, in which diamond abrasive grains are embedded, to the edge of an opening of a glass sheet.
When the glass sheet to be processed is ground using diamond abrasive grains, frictional heat may be generated causing seizure. Thus, coolant flow channels for flowing a coolant (for example, water) therethrough are formed between the diamond abrasive grains and an anchoring layer to which the grains are anchored, and the glass sheet.
Known methods for anchoring diamond abrasive grains to a drill and the like include a Ni electrodeposition method and a metal bonding method. The Ni electrodeposition method is a method in which diamond abrasive grains are secured by nickel plating. For example, a cloth bag filled with diamond abrasive grains is submerged in a nickel plating solution, and a wire penetrating the cloth bag is employed as a cathode so as to energize between the cathode and a nickel anode provided in the plating solution. The wire gradually increases in size while precipitating nickel in the diamond and plating solution. At this time, the diamond abrasive grains are captured in the nickel film so as to be lightly anchored to the surface of the wire. While this plating wire is slowly being wound, the aforementioned energization is continuously performed. The wire protruded from the cloth bag is subsequently plated in the plating solution until the precipitated nickel has a predetermined thickness.
The metal bonding method is a method in which metal powder and diamond abrasive grains are mixed and then heated to thereby sinter the metal powder, so that the diamond abrasive grains are anchored to the metal as being partially embedded in the metal.
FIG. 7 is a schematic cross-sectional view illustrating a diamond bonded portion of a sheet glass tool forced by a Ni electrodeposition method or a metal bonding method. A diamond abrasive grain 101 is anchored in a Ni electrodeposition layer (metal bonded layer) 102 and protruded from the Ni electrodeposition layer (metal bonded layer) 102. The tip of the diamond abrasive grain 101 protruded from the Ni electrodeposition layer (metal bonded layer) 102 is in contact with a glass sheet 103, and a coolant flow channel CL is formed between the Ni electrodeposition layer (metal bonded layer) 102 and the glass sheet 103.
In grinding a glass sheet using the sheet glass tool, the coolant is supplied into the coolant flow channel CL, thereby removing frictional heat during the grinding and thus preventing seizure. The coolant flow channel CL also serves to discharge chippings produced during the grinding and diamond abrasive grains dislodged from the tool.